# Overpressurization of Natural Gas Distribution System, Explosions, and Fires - **operation:** document - **citation:** PLD18MR003 - **title:** Overpressurization of Natural Gas Distribution System, Explosions, and Fires - **source type:** incident - **agency:** National Transportation Safety Board - **status:** current - **official:** true - **published on:** 2019-09-24 - **effective on:** 2018-09-13 - **summary:** Accident. in Merrimack Valley, MA, USA. on 2018-09-13. Columbia Gas of Massachusetts. Leak/explosion/fire - **machine formats:** - **json:** https://regulus.evalyn.ai/document/ntsb-case-pld18mr003.json - **markdown:** https://regulus.evalyn.ai/document/ntsb-case-pld18mr003.md - **app url:** https://regulus.evalyn.ai/document/ntsb-case-pld18mr003 - **source url:** https://www.ntsb.gov/investigations/Pages/PLD18MR003.aspx **body:** NTSB investigation PLD18MR003. Event Type: Accident Event Date: 2018-09-13 Event City: Merrimack Valley Event State Or Region: MA Event Country: USA Pipeline Operator: Columbia Gas of Massachusetts Pipeline Type: Distribution Accident Type: Leak/explosion/fire Completion Status: Completed Report Number: PAR1902 Report Date: 2019-09-24 Probable cause: The National Transportation Safety Board determines that the probable cause of the overpressurization of the natural gas distribution system and the resulting fires and explosions was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main. Contributing to the accident was a low-pressure natural gas distribution system designed and operated without adequate overpressure protection. Tier1Name: System maintenance Tier2Name: Pressure/flow control malf/failure Tier1Name: Emergency response Tier2Name: Fire (post-release) Tier1Name: System shutdown Tier2Name: Emergency shutoff Tier1Name: System operating, changing flow/pressure Tier2Name: Servicing event Tier1Name: System maintenance Tier2Name: Product leak/release Tier1Name: Initiating product flow Tier2Name: System disabled Tier1Name: System maintenance Tier2Name: Servicing event Finding Tier1Name: Organizational Finding Tier2Name: Management Finding Tier3Name: Policy/procedure Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Management - Policy/procedure - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Documentation/record keeping Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Support/oversight/monitoring - Documentation/record keeping - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Management Finding Tier3Name: Resources Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Management - Resources - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Safety programs Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Support/oversight/monitoring - Safety programs - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Oversight Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Support/oversight/monitoring - Oversight - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Management Finding Tier3Name: Communication (organizational) Finding Modifier Name: State/Local agency Finding Report Text: Organizational - Management - Communication (organizational) - State/Local agency Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Enforcement Finding Modifier Name: State/Local agency Finding Report Text: Organizational - Support/oversight/monitoring - Enforcement - State/Local agency Finding Tier1Name: Organizational Finding Tier2Name: Development Finding Tier3Name: Design Finding Modifier Name: Not specified Finding Report Text: Organizational - Development - Design - Not specified Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline operation/capability Finding Tier3Name: Pipeline temp/pressure/flow parameter(s) Finding Modifier Name: Incorrect service/maintenance Finding Report Text: Pipeline - Pipeline operation/capability - Pipeline temp/pressure/flow parameter(s) - Incorrect service/maintenance Finding Tier1Name: Personnel Finding Tier2Name: Experience/knowledge Finding Tier3Name: Experience/qualifications Finding Modifier Name: Pipeline maintenance personnel Finding Report Text: Personnel - Experience/knowledge - Experience/qualifications - Pipeline maintenance personnel Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline handling/service Finding Tier3Name: Servicing Finding Modifier Name: Related records Finding Report Text: Pipeline - Pipeline handling/service - Servicing - Related records Finding Tier1Name: Personnel Finding Tier2Name: Experience/knowledge Finding Tier3Name: Experience/qualifications Finding Modifier Name: Pipeline maintenance personnel Finding Report Text: Personnel - Experience/knowledge - Experience/qualifications - Pipeline maintenance personnel Official NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations. What Happened On September 13, 2018, about 4:00 p.m. local time, a series of structure fires andexplosions occurred after high-pressure natural gas was released into a low-pressure natural gas distribution system in the northeast region of the Merrimack Valley in the Commonwealth of Massachusetts. The natural gas distribution system was owned and operated by Columbia Gas of Massachusetts (CMA), a subsidiary of NiSource, Inc. CMA delivers natural gas to about 325,000 customers in Massachusetts. The fires and explosions damaged 131 structures, including at least 5 homes that were destroyed in the city of Lawrence and the towns of Andover and North Andover. Most of the damage occurred from fires ignited by natural gas-fueled appliances; several of the homes were destroyed by natural gas-fueled explosions. Fire departments from the three municipalities were dispatched to the fires and explosions. First responders initiated the Massachusetts fire-mobilization plan and received mutual aid from neighboring districts in Massachusetts, New Hampshire, and Maine. Emergency management officials had National Grid United States (NG) (the electric utility) shut down electrical power in the area, the state police closed local roads, and freight and passenger railroad operations in the area were suspended. CMA shut down the low-pressure natural gas distribution system, affecting 10,894 customers, including some outside the affected area who had their service shut off as a precaution. What We Found We determined that the probable cause of the overpressurization of the natural gas distribution system and the resulting fires and explosions was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main. Contributing to the accident was a low-pressure natural gas distribution system designed and operated without adequate overpressure protection. What We Recommended We made recommendations to the Pipeline and Hazardous Materials Safety Administration, To the States of Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Florida, Georgia, Idaho, Illinois, Iowa, Kentucky, Louisiana, Maine, Maryland, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Mexico, New York, North Carolina, Ohio, Pennsylvania, South Carolina, South Dakota, Texas, Utah, Virginia, Wisconsin, and Wyoming, To the Commonwealth of Massachusetts Executive Office of Public Safety and Security and To NiSource, Inc. PAR-19-02 <<>> Overpressurization of Natural Gas Distribution System, Explosions, and Fires in Merrimack Valley, Massachusetts September 13, 2018 Accident Report NTSB/PAR-19/02 PB2019-101365 National Transportation Safety Board <<>> NTSB/PAR-19/02 PB2019-101365 Notation 59529 Adopted September 24, 2019 Pipeline Accident Report Overpressurization of Natural Gas Distribution System, Explosions, and Fires in Merrimack Valley, Massachusetts September 13, 2018 National Transportation Safety Board 490 L’Enfant Plaza, S.W. Washington, D.C. 20594 <<>> National Transportation Safety Board. 2019. Overpressurization of Natural Gas Distribution System, Explosions, and Fires in Merrimack Valley, Massachusetts, September 13, 2018. Pipeline Accident Report NTSB/PAR-19/02. Washington, DC. Abstract: On September 13, 2018, about 4:00 p.m. local time, a series of structure fires and explosions occurred after high-pressure natural gas was released into a low-pressure natural gas distribution system in the northeast region of the Merrimack Valley in the Commonwealth of Massachusetts. The natural gas distribution system was owned and operated by Columbia Gas of Massachusetts, a subsidiary of NiSource, Inc. Columbia Gas of Massachusetts delivers natural gas to about 325,000 customers in Massachusetts. One person was killed and 22 individuals, including three firefighters, were transported to local hospitals due to injuries; seven other firefighters incurred minor injuries. The fires and explosions damaged 131 structures, including at least 5 homes that were destroyed in the city of Lawrence and the towns of Andover and North Andover. Most of the damage occurred from fires ignited by natural gas-fueled appliances; several of the homes were destroyed by natural gas-fueled explosions. Fire departments from the three municipalities were dispatched to the fires and explosions. First responders initiated the Massachusetts fire- mobilization plan and received mutual aid from neighboring districts in Massachusetts, New Hampshire, and Maine. Emergency management officials had the electric utility shut down electrical power in the area, the state police closed local roads, and freight and passenger railroad operations in the area were suspended. Columbia Gas of Massachusetts shut down the low-pressure natural gas distribution system, affecting 10,894 customers, including some outside the area who had their service shut off as a precaution. The National Transportation Safety Board made new recommendations to the Pipeline and Hazardous Materials Safety Administration; the 34 states with an industrial exemption for natural gas infrastructure projects; the Commonwealth of Massachusetts Executive Office of Public Safety and Security; and NiSource, Inc. The National Transportation Safety Board (NTSB) is an independent federal agency dedicated to promoting aviation, railroad, highway, marine, and pipeline safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board Act of 1974 to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The NTSB makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and statistical reviews. The NTSB does not assign fault or blame for an accident or incident; rather, as specified by NTSB regulation, “accident/incident investigations are fact-finding proceedings with no formal issues and no adverse parties … and are not conducted for the purpose of determining the rights or liabilities of any person.” 49 C.F.R. § 831.4. Assignment of fault or legal liability is not relevant to the NTSB’s statutory mission to improve transportation safety by investigating accidents and incidents and issuing safety recommendations. In addition, statutory language prohibits the admission into evidence or use of any part of an NTSB report related to an accident in a civil action for damages resulting from a matter mentioned in the report. 49 U.S.C. § 1154(b). For more detailed background information on this report, visit the NTSB investigation website and search for NTSB accident ID PLD18MR003. Recent publications are available in their entirety on the Internet at the NTSB website. Other information about available publications also may be obtained from the website or by contacting: National Transportation Safety Board Records Management Division, CIO-40, 490 L’Enfant Plaza, SW, Washington, DC 20594, (800) 877-6799 or (202) 314-6551. NTSB publications may be purchased from the National Technical Information Service. To purchase this publication, order product number PB2019-101308 from: National Technical Information Service, 5301 Shawnee Rd., Alexandria, VA 22312, (800) 553-6847 or (703) 605-6000, NTIS website. NOTE: This report was reissued on November 7, 2019, with corrections to page 60 to remove NiSource employee information. <<>> NOTE: This report was reissued on November 20, 2019, with corrections to pages 30 and 31 to add a citation to Table 4. NOTE: This report was reissued on April 30, 2021, with corrections to pages 29-31, 43-44, and 50. <<>> Contents Figures ........................................................................................................................................... iii Tables ............................................................................................................................................ iv Acronyms and Abbreviations .......................................................................................................v Executive Summary .................................................................................................................... vii 1. Factual Information .................................................................................................................1 1.1 Accident Synopsis ....................................................................................................................1 1.2 Background ..............................................................................................................................3 1.2.1 NiSource ........................................................................................................................3 1.2.2 Feeney Brothers .............................................................................................................4 1.2.3 Natural Gas Distribution Systems ..................................................................................4 1.3 Events Preceding the Overpressure ..........................................................................................7 1.4 Emergency Response ...............................................................................................................9 1.4.1 Local and State Response ..............................................................................................9 1.4.2 Columbia Gas Response ..............................................................................................11 1.4.3 Community Impact ......................................................................................................13 1.5 Natural Gas Main Replacement Project .................................................................................13 1.5.1 Scope ............................................................................................................................13 1.5.2 Project Reviews ...........................................................................................................15 1.5.3 Sensing Line Documentation .......................................................................................16 1.6 Engineering Project Management ..........................................................................................18 1.6.1 Staffing and Scope of Responsibilities ........................................................................18 1.6.2 Measurement and Regulation Department ...................................................................20 1.7 Overpressure Protection .........................................................................................................22 1.7.1 Overpressurization Protection Requirements ..............................................................22 1.7.2 Previous Overpressurization Accidents Investigated by the NTSB ............................23 1.7.3 Previous NiSource Overpressurization Incidents ........................................................25 1.8 Pipeline Safety Management Systems ...................................................................................26 1.9 Professional Engineer Review and Approval .........................................................................28 1.10 Government Oversight ...........................................................................................................31 1.10.1 Federal Oversight .........................................................................................................31 1.10.2 Massachusetts Oversight ..............................................................................................31 2 Postaccident Actions ..............................................................................................................33 2.1 NTSB Safety Recommendation to Commonwealth of Massachusetts ..................................33 2.2 NTSB Urgent Recommendations to NiSource .......................................................................34 2.3 NiSource Emergency Preparedness and Response Actions ...................................................36 2.4 Industry Actions .....................................................................................................................38 <<>> NTSB Pipeline Accident Report 3 Analysis ...................................................................................................................................39 3.1 Exclusions ..............................................................................................................................39 3.2 Overpressurization Protection for Low-Pressure Natural Gas Systems .................................39 3.3 CMA Engineering Processes ..................................................................................................40 3.3.1 Records and Documentation ........................................................................................41 3.3.2 Constructability Review...............................................................................................42 3.3.3 Engineering Risk Assessment ......................................................................................43 3.4 Professional Engineer Review and Approval .........................................................................43 3.5 Emergency Response .............................................................................................................44 3.5.1 Public Safety Answering Points...................................................................................44 3.5.2 Emergency Responder Communications .....................................................................44 3.5.3 NiSource Emergency Coordination with Municipal Responders ................................45 4 Conclusions .............................................................................................................................48 4.1 Findings ..................................................................................................................................48 4.2 Probable Cause .......................................................................................................................49 5 Recommendations ..................................................................................................................50 5.1 New Recommendations ..........................................................................................................50 5.2 Previously Issued Recommendations .....................................................................................51 Appendix .......................................................................................................................................52 Appendix A. The Investigation ......................................................................................................53 Appendix B. NiSource Safety Management System Plan .............................................................54 Appendix C. Enforcement Actions ................................................................................................55 Appendix D. Constructability Safety Review ................................................................................57 Appendix E. NiSource Operational Notice ON 15-05...................................................................59 References .....................................................................................................................................62 ii <<>> NTSB Pipeline Accident Report Figures Figure 1. Map of the damaged structures in the area impacted by the overpressurization. ........... 2 Figure 2. Remnants of house where the fatality and two severe injuries occurred. ....................... 3 Figure 3. Typical configuration of the Merrimack Valley low-pressure natural gas distribution system. ............................................................................................................................................ 5 Figure 4. Typical configuration of high-pressure natural gas distribution system installed postaccident..................................................................................................................................... 7 Figure 5. Salem Street tie-in for the South Union Street project ................................................... 8 Figure 6. Location of September 13, 2018, tie-in and the Winthrop Avenue regulator station. .... 9 Figure 7. Areas along South Union Street with tie-ins impacted by the project.......................... 14 Figure 8. The Winthrop Avenue regulator station. ...................................................................... 15 Figure 9. Pipeline SMS maturity model. ...................................................................................... 27 Figure 10. NiSource Safety Management System Plan Part A. ................................................... 54 Figure 11. NiSource Safety Management System Plan Part B. ................................................... 54 iii <<>> NTSB Pipeline Accident Report Tables Table 1. Local fire response. ........................................................................................................ 10 Table 2. Required CMA project workflow documentation. ......................................................... 15 Table 3. Sources of sensing line information and select regulator station documentation. ......... 17 Table 4. P.E. industrial exemption for infrastructure project practices. ....................................... 30 Table 5. Massachusetts DPU enforcement actions for the 5 years previous to the accident. ...... 55 iv <<>> NTSB Pipeline Accident Report Acronyms and Abbreviations AGA American Gas Association ANSI American National Standards Institutes API American Petroleum Institute ASME American Society of Mechanical Engineers CFR Code of Federal Regulations CMA Columbia Gas of Massachusetts CoMIRS Commonwealth of Massachusetts Interoperable Radio System DPU Massachusetts Department of Public Utilities EOC emergency operations center ERP emergency response plans FMEA failure modes and effects analysis FOL field operations leader GIS geographic information system IC incident commander LFE leader of field engineering M&R Measurement and Regulation MAOP maximum allowable operating pressure MEMA Massachusetts Emergency Management Agency MOC management of change NG National Grid United States NCEES National Council of Examiners for Engineering and Surveying NSPE National Society of Professional Engineers NTSB National Transportation Safety Board v <<>> NTSB Pipeline Accident Report ON Operational Notice OQ operator qualification P.E. professional engineer PHMSA Pipeline and Hazardous Materials Safety Administration PSAP Public Safety Answering Points psi pounds per square inch psig pounds per square inch, gauge PSMS Pipeline Safety Management Systems RP Recommended Practice RSPA Research and Special Programs Administration SCADA Supervisory Control and Data Acquisition system SCIP Statewide Communications Interoperability Plan SMS safety management systems w.c. water column WMS work management system vi <<>> NTSB Pipeline Accident Report Executive Summary On September 13, 2018, about 4:00 p.m. local time, a series of structure fires and explosions occurred after high-pressure natural gas was released into a low-pressure natural gas distribution system in the northeast region of the Merrimack Valley in the Commonwealth of Massachusetts. The natural gas distribution system was owned and operated by Columbia Gas of Massachusetts, a subsidiary of NiSource, Inc. Columbia Gas of Massachusetts delivers natural gas to about 325,000 customers in Massachusetts. One person was killed and 22 individuals, including three firefighters, were transported to local hospitals due to injuries; seven other firefighters incurred minor injuries. The fires and explosions damaged 131 structures, including at least 5 homes that were destroyed in the city of Lawrence and the towns of Andover and North Andover. Most of the damage occurred from fires ignited by natural gas-fueled appliances; several of the homes were destroyed by natural gas-fueled explosions. Fire departments from the three municipalities were dispatched to the fires and explosions. First responders initiated the Massachusetts fire-mobilization plan and received mutual aid from neighboring districts in Massachusetts, New Hampshire, and Maine. Emergency management officials had National Grid United States (the electric utility) shut down electrical power in the area, the state police closed local roads, and freight and passenger railroad operations in the area were suspended. Columbia Gas of Massachusetts shut down the low-pressure natural gas distribution system, affecting 10,894 customers, including some outside the area who had their service shut off as a precaution. The accident investigation focused on the following safety issues: • Adequacy of natural gas regulations • Project documentation • Constructability review • Project management • Risk assessment • Safety management systems • Licensed professional engineer approval of natural gas projects • Emergency response The National Transportation Safety Board determines that the probable cause of the overpressurization of the natural gas distribution system and the resulting fires and explosions was Columbia Gas of Massachusetts’ weak engineering management that did not adequately plan, review, sequence, and oversee the construction project that led to the abandonment of a cast iron main without first relocating regulator sensing lines to the new polyethylene main. Contributing to the accident was a low-pressure natural gas distribution system designed and operated without adequate overpressure protection. vii <<>> NTSB Pipeline Accident Report 1. Factual Information 1.1 Accident Synopsis On September 13, 2018, about 4:00 p.m. local time, a series of structure fires and explosions occurred after high-pressure natural gas was released into a low-pressure natural gas distribution system in the northeast region of the Merrimack Valley in the Commonwealth of Massachusetts. The natural gas distribution system was owned and operated by Columbia Gas of Massachusetts (CMA), a subsidiary of NiSource, Inc. CMA delivers natural gas to about 325,000 customers in Massachusetts. The fires and explosions damaged 131 structures, including at least 5 homes that were destroyed in the city of Lawrence and the towns of Andover and North Andover. (See figure 1.) Most of the damage occurred from fires ignited by natural gas-fueled appliances; several of the homes were destroyed by natural gas-fueled explosions. Fire departments from the three municipalities were dispatched to the fires and explosions. First responders initiated the Massachusetts fire-mobilization plan and received mutual aid from neighboring districts in Massachusetts, New Hampshire, and Maine. Emergency management officials had National Grid United States (NG) (the electric utility) shut down electrical power in the area, the state police closed local roads, and freight and passenger railroad operations in the area were suspended. CMA shut down the low-pressure natural gas distribution system, affecting 10,894 customers, including some outside the affected area who had their service shut off as a precaution. 1 <<>> NTSB Pipeline Accident Report Figure 1. Map of the damaged structures in the area impacted by the overpressurization. An 18-year-old male was killed when a home exploded, and the chimney fell onto the vehicle where he was sitting. (See figure 2.) Another person in the vehicle at the time of the 2 <<>> NTSB Pipeline Accident Report explosion was seriously injured, as was someone on the second floor of the house. In total, 22 people, including 3 firefighters, were transported to hospitals for treatment of their injuries. Injuries included respiratory injuries related to smoke inhalation from fires and musculoskeletal injuries from evacuating. Some people were transported to hospitals to maintain ongoing medical treatment that could not be continued in their homes because of the shutdown of natural gas and electricity and the evacuation of residents. Figure 2. Remnants of house where the fatality and two severe injuries occurred. 1.2 Background 1.2.1 NiSource NiSource, Inc. is an Indiana-based energy holding company whose subsidiaries are regulated natural gas and electric utility companies serving about 3.9 million customers in seven states.1 Its natural gas distribution operations comprise about 60,000 miles of pipeline and include 732 low-pressure natural gas distribution systems. NiSource’s Massachusetts subsidiary, CMA, 1 NiSource is the successor to a corporation organized in 1987 under the name of Northern Indiana Public Service Company Industries, Inc., which changed its name to NiSource in 1999. 3 <<>> NTSB Pipeline Accident Report delivers natural gas to over 325,000 natural gas customers in southeastern Massachusetts, the greater Springfield area, and the Merrimack Valley.2 1.2.2 Feeney Brothers CMA contracted with Feeney Brothers, a pipeline services firm, to work on a CMA project to replace an existing cast iron main with a polyethylene main. 3 About 7:00 a.m. on the day of the accident, a CMA construction coordinator, along with four employees of Feeney Brothers, arrived at Salem and South Union Streets in Lawrence, Massachusetts, to continue work on this project. The work they performed that day led to the overpressurization of the natural gas distribution system. All crewmembers were trained and qualified in accordance with the Pipeline Operator Qualification Rule, commonly known as OQ.4 Following the accident, the contractor crewmembers, along with the CMA construction coordinator, were alcohol and drug tested in accordance with Title 49 Code of Federal Regulations (CFR) Part 199. The test results were negative for alcohol or other drugs. 1.2.3 Natural Gas Distribution Systems Natural gas distribution systems deliver natural gas to customers for heating, cooking, lighting, and other uses. A basic distribution system has three elements: (1) natural gas mains that transport natural gas underground, (2) service lines that deliver natural gas from the mains to customers, and (3) meters that measure the quantity of natural gas used by each customer. Customer piping takes natural gas from the meter to customer’s appliances where it is used. To minimize service interruptions, normal maintenance and natural gas distribution system upgrades are typically performed with the system operating. Both low-pressure and high-pressure natural gas distribution systems are used to supply natural gas to customers. In a low-pressure natural gas distribution system, the natural gas in the mains is essentially the same pressure as the pressure provided to the customer’s piping and used by the appliances. Natural gas is typically supplied to the mains from a high-pressure source through a regulator station that reduces the pressure to that required by the customers. The low-pressure natural gas distribution system in the Merrimack Valley was installed in the early 1900s with cast iron mains. The system used 14 regulator stations to supply natural gas to the mains and control pressure.5 The regulator stations each contained two regulators in series⸻a worker regulator and a monitor regulator⸻each with a sensing line that feeds back the pressure in the main to the regulator, forming a redundant closed-loop control system. The worker regulator is the primary regulator that maintains the natural gas pressure, and the monitor regulator provides a redundant backup to the worker regulator. Each of the regulator stations reduced the natural gas 2 Although CMA had internal guidance documents specifically for its employees, NiSource also had guidance documents that employees in all its subsidiaries were required to follow. In this report, guidance documents are identified accordingly. 3 Feeney Brothers, a utility services firm headquartered in Dorchester, Massachusetts, was established in 1988 and employs over 700 employees and operates throughout Massachusetts, Connecticut, and New York. 4 Title 49 Code of Federal Regulations (CFR) Part 192, Subpart N. 5 Regulator stations house the worker and monitor regulators that are used to maintain natural gas pressure. 4 <<>> NTSB Pipeline Accident Report pressure from about 75 pounds per square inch gauge (psig) to 12 inches of water column (w.c.), about 0.5 psig, for distribution through the mains and delivery to customers.6 Since the regulator stations are the primary means of pressure control in the low-pressure systems, an overpressure condition in a natural gas distribution system could affect all customers served by the system. This is an inherent weakness of a low-pressure natural gas distribution system. Figure 3 shows a typical arrangement for the low-pressure natural gas distribution system used in the Merrimack Valley before the accident. Figure 3. Typical configuration of the Merrimack Valley low-pressure natural gas distribution system. 6 In the pipeline industry, it is customary to measure anything less than 1 psig in inches of water column. A measurement of 1 inch w.c. equals 0.0361 psig. 5 <<>> NTSB Pipeline Accident Report In a high-pressure natural gas distribution system, the natural gas pressure in the main is substantially higher than that required by the customer. A pressure regulator is installed at each meter to reduce the pressure. These regulators incorporate an overpressure protection device to prevent overpressurization of the customer’s piping and appliances should the regulator fail. Additionally, excess flow valves are installed in the service line.7 Because each customer’s service in a high-pressure natural gas distribution system is protected by an excess flow valve and a pressure regulator, it is highly unlikely that an overpressure condition in the main would impact multiple customers. Figure 4 shows a typical high-pressure natural gas distribution system. This is the type of natural gas distribution system that was installed postaccident in the Merrimack Valley. 7 An excess flow valve is a mechanical safety device installed on a gas service line to a residence or small commercial gas customer. In the event of damage to the gas service line between the street and the meter, the excess flow valve will minimize the flow of gas through the service line. Current federal regulations require a gas distribution company to install such a device on new or replacement service lines for single-family residences and certain multifamily and commercial buildings where the service line pressure is above 10 psig. See 49 CFR 192.383 for specific requirements. 6 <<>> NTSB Pipeline Accident Report Figure 4. Typical configuration of high-pressure natural gas distribution system installed postaccident. 1.3 Events Preceding the Overpressure About 7:00 a.m. on the day of the accident, a CMA construction coordinator, along with four employees of Feeney Brothers, arrived at Salem and South Union Streets in Lawrence, Massachusetts, to continue work on a CMA project to replace an existing cast iron main with a polyethylene main. The crew completed the installation according to the CMA work plan, placed the new tie-ins into service, and isolated the existing cast iron main shortly before 4:00 p.m., by closing 7 <<>> NTSB Pipeline Accident Report valves on a 2-inch plastic bypass pipe between the cast iron and polyethylene mains. 8 The crew then cut the bypass pipe to abandon the cast iron main. (See figure 5.) In postaccident interviews, crewmembers said that within minutes of closing the valves and cutting the bypass, they observed the pressure gauge on the polyethylene main exceed the expected readings.9 Furthermore, a fitting on the polyethylene natural gas main blew off into the hand of one of the workers. The crewmembers said that they responded quickly to plug the blowing natural gas, and they heard emergency vehicles in the neighborhood and observed smoke plumes in multiple directions within minutes. 2-inch bypass with valve at each end Pressure gauges 6-inch main fed by newly energized 8-inch plastic main connected to Winthrop Avenue regulator station Figure 5. Salem Street tie-in for the South Union Street project (looking west). Photograph courtesy of Feeney Brothers. At the Winthrop Avenue regulator station, about 0.5 mile south of the work area, the abandoned cast iron main was still connected to the regulator sensing lines providing input to the two pressure regulators used to control the system pressure. 10 (See figure 6.) Once the contractor 8 A tie-in involves connecting new piping to existing piping. In this case, the main ran north and south while the branches ran east and west. When the main was replaced, the east and west branches needed to be tied into the new main. 9 Supporting documentation referenced in this report can be found in the public docket for this accident, accessible from the NTSB Accident Dockets web page by searching PLD18MR003. 10 Sensing lines are also called control lines or static lines. 8 <<>> NTSB Pipeline Accident Report crew isolated the cast iron main, the natural gas pressure began to drop in the cast iron main and in the regulator sensing lines. As the pressure dropped, the pressure regulators responded by opening further, increasing pressure in the natural gas distribution system. Since the Winthrop Avenue regulators no longer sensed system pressure, they fully opened, allowing high-pressure natural gas to be released into the low-pressure natural gas distribution system. Figure 6. Location of September 13, 2018, tie-in and the Winthrop Avenue regulator station. 1.4 Emergency Response 1.4.1 Local and State Response The overpressurization of the low-pressure natural gas distribution system in the Merrimack Valley impacted over 10,000 natural gas customers in three municipalities⸻Lawrence, North Andover, and Andover. The emergency call centers in these municipalities, known as public safety answering points (PSAP) began receiving 9-1-1 calls immediately following the overpressurization from residents and businesses reporting fires and explosions and requesting assistance. Shortly after 4:00 p.m., the fire departments in Lawrence, North Andover, and Andover were inundated with emergency calls reporting structure fires and explosions. Within the first 30 minutes, all three fire departments had exhausted their list of mutual aid. The incident commanders (IC) fro - **truncated:** true - **body characters:** 195962